1. Field of the Invention
The present invention relates to a leakage current cut-off device for a ternary content addressable memory.
2. Description of the Prior Art
A Ternary Content Addressable Memory (TCAM) is grouped into several array segments, and each segment contains an array of TCAM cell pairs. Each TCAM cell pair is a bit and contains a storage cell and a don't-care cell. The storage value of each bit may be a “0” state, an “1” state or an “X” state that is also called don't-care state. One terminal of the storage cell connects to an electrical power source for receiving a voltage, and the other terminal connects to the ground for discharging.
A TCAM may be in read operation, write operation and search operation or standby mode. And, it is not necessary to compare the storage data of a bit with the inquiry data when “X” state is set.
The storage cell still connects to between the electrical power source and the ground via a charging terminal and a discharging terminal, respectively, and that will be accompanied by a leakage current. The leakage current consumes the electrical power and furthermore decreases the device reliability, so the leakage current must be reduced to as small as possible.
As shown in FIG. 1 for a conventional technique, n-channel metal oxide semiconductor (NMOS) field effect transistors MA, MB implements the data-retention power gating technique to reduce the leakage current, wherein MB is diode connected and also called a diode-connected NMOS. When the NMOS MA is turned on and the TCAM storage cell is likely connected to the ground to support the full speed operation; and when the NMOS MA is turned off and the TCAM storage cell is connected to the ground through the NMOS MB. The leakage current charges the discharging terminal of the TCAM storage cell, the virtual ground (VGND), to reach a saturated voltage via the NMOS MB. Therefore, the voltage difference between the charging terminal and the discharging terminal will be decreased, and the leakage current is also reduced.
This kind of structure is good enough for the Synchronous Dynamic Random Access Memory (SDRAM), but, for the TCAM, it would not be satisfied, because “X” state is not considered. The technique of reducing the leakage current for TCAM has been developed.